The present invention relates to a torque limiting mechanism that couples a power source with a driven apparatus. More particularly, the present invention pertains to a mechanism that disconnects the power source from the driven apparatus when a malfunction occurs in the driven apparatus and consequently the load of the driven apparatus becomes excessive.
Japanese Unexamined Patent Publication No. 8-159028 describes such a torque limiting mechanism that couples a power source with a driven apparatus. This mechanism includes a power receiver and a pulley that are located coaxially with a rotary shaft of the driven apparatus. The pulley is rotatably supported on the driven apparatus by an angular bearing and is coupled to the power source by a belt. The receiver is secured to the rotary shaft and is connected with the pulley by rolling balls. Specifically, the receiver has recesses, and the pulley also has recesses the number of which corresponds to the number of the recesses in the receiver. Each ball is located between the receiver and the pulley and is fitted in a corresponding pair of the recesses. The pulley and the angular bearing are urged toward the receiver by a disk spring. Rotation of the pulley is thus transmitted to the rotary shaft by way of the balls and the receiver. When the load torque of the driven apparatus exceeds a predetermined value, the balls are disengaged from the recesses of the pulley. Accordingly, the pulley is disconnected from the power receiver and the power transmission is stopped.
The balls are located near the periphery of the receiver. Therefore, rotation of the pulley applies centrifugal force to the balls. When the rotation speed of the pulley is high, a great centrifugal force can disengage the balls from the recesses of the power receiver and the pulley. This disconnects the power source from the driven apparatus even if the load torque of the driven apparatus is below the predetermined value.
The value of load torque of the driven apparatus at which the torque limiting mechanism disconnects the power source from the driven apparatus is determined by the frictional force between the balls and the recesses of the pulley, and the force of the disc spring. In other words, the coefficient of friction between the balls and the recesses of the pulley must be taken into account when determining the load torque value at which the power source is disconnected from the driven apparatus. Therefore, it is difficult to stabilize the disconnection characteristics of the torque limiting mechanism. That is, the mechanism may disconnect the power source from the driven apparatus when the load torque applied is smaller or greater than the predetermined value.